The building has stone cladding and graffiti on stone has to be removed quite quickly because after a while it is nearly impossible to clean it.

Don't get me wrong it is still pr disaster and it should be handled differently, the cover of it, standing policemans around... It just gives feeling that they don't want anyone to see it (which was maybe someones intention). Leaving it visible with some press release about protecting stone buildings and cleaning it immediately afterwards would be better pr option.